I have set up two Mongo instances in EC2 having RAID10. I set up replica set using those two servers and found that this replica is very very slow over network. If I log in locally and issue query inside mongo console all is fine. I tested disk speed no issue here:

1 Answer
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This is a bit light on details - you don't mention some vital information like what exactly is slow (is this access via a driver for example). Replication adds a little load to the primary, but in general is pretty lightweight.

What size instances are you running, and where (replicating across regions etc.)? You might be saturating your available bandwidth when you add the replication data to the mix. I would only expect that though if you were running a write heavy application or if perhaps you were doing slaveOK reads.

Finally, as a general guide to running MongoDB in AWS, check out this white paper:

I was looking for the issue for long time now, I know its light on details. Thanks for your help ! I have to time queries on our application side to know the answer, seems that mongo is ok.
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LukaszMar 13 '12 at 8:21

From this comment, it seems that this question is no longer an open one, maybe consider closing it?
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Mike FiedlerApr 22 '12 at 12:46